BSI Image Sensor and Method of Forming Same

ABSTRACT

A backside illumination (BSI) image sensor and a method of forming the same are provided. A method includes forming a plurality of photosensitive pixels in a substrate, the substrate having a first surface and a second surface, the second surface being opposite the first surface, the substrate having one or more active devices on the first surface. A first portion of the second surface is protected. A second portion of the second surface is patterned to form recesses in the substrate. An anti-reflective layer is formed on sidewalls of the recesses. A metal grid is formed over the second portion of the second surface, the anti-reflective layer being interposed between the substrate and the metal grid.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a division of U.S. application Ser. No. 15/079,886,entitled “BSI Image Sensor and Method of Forming Same,” filed on Mar.24, 2016, which application is hereby incorporated herein by reference.

BACKGROUND

Backside illumination (BSI) image sensor chips are replacing front-sideillumination sensor chips for their higher efficiency in capturingphotons. In the formation of the BSI image sensor chips, image sensorsand logic circuits are formed in a semiconductor substrate of a wafer,followed by the formation of an interconnect structure on a front sideof the silicon chip.

The image sensors in the BSI image sensor chips generate electricalsignals in response to the stimulation of photons. The magnitudes of theelectrical signals (such as the photo-current) depend on the intensityof the incident light received by the respective image sensors. Theimage sensors, however, suffer from non-optically generated signals,which include the leakage signals, thermally generated signals, darkcurrents, and the like. Accordingly, the electrical signals generated bythe image sensors need to be calibrated, so that the undesirable signalsare cancelled out from the output signals of the image sensors. Tocancel the non-optically generated signals, black reference imagesensors are formed, and are used to generate non-optically generatedsignals. The black reference image sensors, therefore, need to beblocked from receiving light signals.

The black reference image sensors are covered by a metal shield, whichis formed on the backside of the semiconductor substrate, in which theimage sensors are formed. Furthermore, backside metal pads are alsoformed on the backside of the semiconductor substrate for bonding ortesting.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-8 are cross-sectional views of various processing steps duringfabrication of a BSI image sensor in accordance with some embodiments.

FIGS. 9A-13B are scanning electron microscope (SEM) images illustratingperspective and cross-sectional views of high absorption structures inaccordance with some embodiments.

FIG. 14 illustrates reflectances of back surfaces of substrates inaccordance with some embodiments.

FIG. 15 is a flow diagram illustrating a method of forming a BSI imagesensor in accordance with some embodiments.

FIGS. 16-19 are cross-sectional views of various processing steps duringfabrication of a BSI image sensor in accordance with some embodiments.

FIG. 20 is a flow diagram illustrating a method of forming a BSI imagesensor in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

A backside illumination (BSI) image sensor and a method of forming thesame are provided in accordance with various exemplary embodiments.Various embodiments include a high absorption (HA) structure on abackside of a BSI sensor to reduce reflectance and to improve absorptionof incident electromagnetic radiation. Various embodiments furtheroptimize process steps performed on a backside of a BSI image sensor toimprove the optical performance of the resulting BSI image sensor. Forexample, various embodiments described herein reduce the optical path ofthe resulting BSI image sensor, which advantageous improves the quantumefficiency and reduces the optical cross-talk of the resulting BSI imagesensor.

FIGS. 1-8 are cross-sectional views of various processing steps duringfabrication of a BSI image sensor 100 in accordance with someembodiments. FIG. 1 illustrates an intermediate structure of the BSIimage sensor 100 in accordance with some embodiments. In someembodiments, the BSI image sensor 100, which may be a part of anun-singulated wafer that comprises additional BSI image sensors similarto the BSI image sensor 100. In the illustrated embodiments, a portionof the BSI image sensor 100 having a pixel array region 103 and a blacklevel correction (BLC) region 105 is illustrated. The BSI image sensor100 may further include other regions such as, for example, a contactpad (E-pad) region and an alignment region such as a scribe-line primarymark (SPM) region, which are not explicitly illustrated, since theirinclusion is not necessary for understanding various embodimentsdescribed herein.

In some embodiments, the BSI image sensor 100 includes a substrate 107.The substrate 107 may comprise, for example, bulk silicon, doped orundoped, or an active layer of a semiconductor-on-insulator (SOI)substrate. Generally, an SOI substrate comprises a layer of asemiconductor material, such as silicon, formed on an insulator layer.The insulator layer may be, for example, a buried oxide (BOX) layer or asilicon oxide layer. The insulator layer is provided on a substrate,such as a silicon or glass substrate. Alternatively, the substrate 107may include another elementary semiconductor, such as germanium; acompound semiconductor including silicon carbide, gallium arsenic,gallium phosphide, indium phosphide, indium arsenide, and/or indiumantimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. Othersubstrates, such as multi-layered or gradient substrates, may also beused. Throughout the description, a surface 107A is referred to as afront surface of the substrate 107, and a surface 107B is referred to asa back surface of the substrate 107, which coincides with a back surfaceof the BSI image sensor 100.

A plurality of photosensitive pixels 109 (including 109A and 109B) areformed at the front surface 107A of the substrate 107. Thephotosensitive pixels 109 include respective photosensitive devices (notillustrated), which may be formed, for example, by implanting suitableimpurity ions. In some embodiments, the impurity ions may be implantedin an epitaxial layer (not illustrated) within the substrate 107. Thephotosensitive devices are configured to covert light signals (e.g.,photons) to electrical signals, and may be PN junction photo-diodes, PNPphoto-transistors, NPN photo-transistors, or the like. For example, thephotosensitive devices may include an n-type implantation region formedwithin a p-type semiconductor layer (e.g., at least a portion of thesubstrate 107). In such embodiments, the p-type substrate may isolateand reduce electrical cross-talk between adjacent photo-active regionsof the photosensitive pixels 109. In an embodiment, the photosensitivepixels 109 extend from the front surface 107A of the substrate 107towards the back surface 107B of the substrate 107 and form aphotosensitive pixel array. In some embodiments, the photosensitivepixels 109 form a two-dimensional rectangular array as viewed from top(not illustrated). In some embodiments, each photosensitive pixel 109may further include a transfer gate transistor (not illustrated) and afloating diffusion capacitor (not illustrated). In each photosensitivepixel 109, a first source/drain region the corresponding transfer gatetransistor is electrically coupled to a respective photosensitivedevice, a second source/drain region the corresponding transfer gatetransistor is electrically coupled to a respective floating diffusioncapacitor.

In some embodiments, isolation regions 111 are formed in the substrate107 between neighboring photosensitive pixels 109 to prevent electricalcross-talk between the photosensitive pixels 109. In some embodiments,the isolation regions 111 may include shallow trench isolation (STI)structures. In some embodiments, the STI structures may be formed bypatterning the front surface 107A of the substrate 107 to form trenchesin the substrate 107 and filling the trenches with suitable dielectricmaterials to form the STI structures. In some embodiments, the substrate107 is patterned using suitable photolithography and etching process. Inother embodiments, the isolation regions 111 may include various dopingregions formed using suitable implantation processes.

In some embodiments, the pixel array region 103 of the BSI image sensor100 includes active photosensitive pixels 109A, which are used forgenerating electrical signals from the sensed light. The BLC region 105of the BSI image sensor 100 includes reference photosensitive pixels109B, which are used for generating reference black level signals. Insome embodiments the photosensitive pixels 109A and 109B may be formedusing similar methods, and structures of the photosensitive pixels 109Aand 109B may be identical to each other.

In some embodiments, one or more active and/or passive devices 121(depicted as a single transistor in FIG. 1 for illustrative purposes)are formed on the front surface 107A of the substrate 107 in addition tothe photosensitive pixels 109 comprising the photodiode devices, thetransfer gate transistors, and the floating diffusion capacitors. Theone or more active and/or passive devices 121 may include various N-typemetal-oxide semiconductor (NMOS) and/or P-type metal-oxide semiconductor(PMOS) devices, such as transistors, capacitors, resistors, diodes,photo-diodes, fuses, and the like. One of ordinary skill in the art willappreciate that the above examples are provided for illustrativepurposes only and are not meant to limit the present invention in anymanner. Other circuitry may be used as appropriate for a givenapplication.

An interconnect structure 113 may be formed on the front surface 107A ofthe substrate 107. Interconnect structure 113 may include an inter-layerdielectric (ILD) layer 115 and/or inter-metal dielectric (IMD) layers117 containing conductive features (e.g., conductive lines and viascomprising copper, aluminum, tungsten, combinations thereof, and thelike) formed using any suitable method, such as damascene, dualdamascene, or the like. The ILD 115 and IMDs 117 may include low-kdielectric materials having k values, for example, lower than about 4.0or even 2.0 disposed between such conductive features. In someembodiments, the ILD 115 and IMDs 117 may be made of, for example,phosphosilicate glass (PSG), borophosphosilicate glass (BPSG),fluorosilicate glass (FSG), SiOxCy, Spin-On-Glass, Spin-On-Polymers,silicon carbon material, compounds thereof, composites thereof,combinations thereof, or the like, formed by any suitable method, suchas spinning, chemical vapor deposition (CVD), plasma-enhanced CVD(PECVD), or the like. The interconnect structure electrically connectsvarious active and/or passive devices (e.g., the photosensitive devicesof the photosensitive pixels 109) to form electrical circuits within theBSI image sensor 100. In some embodiments, a passivation layer 119 isformed over the interconnect structure 113. The passivation layer 119may be formed of a non-low-k dielectric material having a k valuegreater than 3.9. In some embodiments, the passivation layer 119 mayinclude one or more layers of silicon oxide, silicon nitride layer, orthe like, and may formed using CVD, PECVD, thermal oxidation, or thelike.

FIGS. 2 and 3 illustrate formation of a high absorption (HA) structure301 on the back surface 107B of the BSI image sensor 100 in the pixelarray region 103. As described below in greater detail, the HA structure301 reduces reflectance of the back surface 107B of the BSI image sensor100. Thus, more light is absorbed by the active photosensitive pixels109A. Referring first to FIG. 2, the BSI image sensor 100 is flipped andbonded to a carrier substrate 201 such that the front surface 107A ofthe substrate 107 faces the carrier substrate 201 and the back surface107B of the substrate 107 is exposed for further processing. Variousbonding techniques may be employed to achieve bonding between the BSIimage sensor 100 and the carrier substrate 201. In some embodiments, thebonding techniques may include for example, a direct bonding processsuch as metal-to-metal bonding (e.g., copper-to-copper bonding),dielectric-to-dielectric bonding (e.g., oxide-to-oxide bonding),metal-to-dielectric bonding (e.g., oxide-to-copper bonding), hybridbonding, adhesive bonding, anodic bonding, any combinations thereofand/or the like. In some embodiments, the carrier substrate 201 mayprovide mechanical support for processing steps performed on the backsurface 107B of the substrate 107. In some embodiments, the carriersubstrate 201 may be formed of silicon or glass and may be free fromelectrical circuitry formed thereon. In such embodiments, the carriersubstrate 201 provides temporary support and is de-bonded from the BSIimage sensor 100 after completing the process steps performed on theback surface 107B of the substrate 107. In other embodiments, thecarrier substrate 201 may comprise a semiconductor substrate (notillustrated), one or more active devices (not illustrated) on thesemiconductor substrate, and an interconnect structure (not illustrated)over the one or more active devices. In such embodiments, in addition toproviding the mechanical support, the carrier substrate 201 may provideadditional electrical functionality to the BSI image sensor 100depending on design requirements.

After the BSI image sensor 100 and the carrier substrate 201 are bonded,a thinning process may be applied to the back surface 107B of the BSIimage sensor 100. In an embodiment, the thinning process serves to allowmore light to pass through from the back surface 107B of substrate 107to photo-active region of the photosensitive pixels 109 without beingabsorbed by the substrate 107. In an embodiment in which thephotosensitive pixels 109 are fabricated in an epitaxial layer, the backsurface 107B of the BSI image sensor 100 may be thinned until theepitaxial layer is exposed. The thinning process may be implemented byusing suitable techniques such as grinding, polishing, a SMARTCUT®procedure, an ELTRAN® procedure, and/or chemical etching. Afterthinning, the substrate 107 may have a thickness of about 2.5 μm, forexample, although other embodiments may include the substrate 107 havinga different thickness after thinning.

Referring further to FIG. 2, a patterned mask 203 is formed on the backsurface 107B of the BSI image sensor 100. In some embodiments, thepatterned mask 203 may formed of a photosensitive material such asphotoresist or a non-photosensitive material such as silicon oxide,silicon nitride, silicon oxinitride, the like, or a combination thereof.In some embodiments in which the patterned mask 203 is formed of aphotosensitive material, the patterned mask 203 is patterned by exposingthe photosensitive material to light. Subsequently, the photosensitivematerial is cured and developed to remove exposed (or unexposed)portions of the photosensitive material. In other embodiments in whichthe patterned mask 203 is formed of a non-photosensitive material, thenon-photosensitive material may be patterned using suitablephotolithography and etching processes.

The patterned mask 203 is patterned to form a plurality of openings 205therein, such that portions of the back surface 107B of the BSI imagesensor 100 in the pixel array region 103 are exposed through theopenings 205, while the back surface 107B of the BSI image sensor 100 inthe BLC region 105 is completely covered by the patterned mask 203. Insome embodiments, unremoved portions 207 of the patterned mask 203 thatare interposed between neighboring openings 205 have a uniform width W₁,a uniform spacing S₁, and a uniform pitch P₁, with P₁ being equal to asum of W₁ and S₁. In some embodiments, the unremoved portions 207 of thepatterned mask 203 may form a two-dimensional rectangular array or atow-dimensional grid as viewed from top. Subsequently, the back surface107B of the BSI image sensor 100 is patterned using a suitable etchingprocess while using the patterned mask 203 as an etch mask. As describedbelow in greater detail, a pattern of the HA structure 301 may be tunedby tuning the width W₁, the spacing S₁, and the pitch P₁ of theunremoved portions 207 of the patterned mask 203. In some embodiments,the width W₁ is between about 0.08 μm and about 0.3 μm, the spacing S₁is between about 0.72 μm and about 2.7 μm, and the pitch P₁ is betweenabout 0.8 μm and about 3 μm.

Referring to FIG. 3, the back surface 107B of the BSI image sensor 100is patterned to form the HA structure 301 on the back surface 107B ofthe BSI image sensor 100. In some embodiments, the back surface 107B ofthe BSI image sensor 100 is patterned using a suitable anisotropic wetetching process, while using the patterned mask 203 as an etch mask. Insome embodiments in which the substrate 107 is formed of silicon, theanisotropic wet etch may be performed using potassium hydroxide (KOH),ethylenediamine pyrocatechol (EDP), tetramethylammonium hydroxide(TMAH), or similar. After forming the HA structure 301, the patternedmask 203 is removed. In some embodiments in which the patterned mask 203is formed of a photoresist, the patterned mask 203 may be removed usingan ashing processes followed by a wet clean process. In otherembodiments in which the patterned mask 203 is formed of anon-photosensitive material, the patterned mask 203 may be removed usinga suitable etching process.

The back surface 107B of the BSI image sensor 100 is patterned to form aplurality of trenches 303 in the substrate 107. In some embodiments, thetrenches 303 may have a depth between about 0.25 μm and about 1 μm. Inthe illustrated embodiment, unremoved portions 305 of the substrate 107that are interposed between the neighboring trenches 303 have azig-zag-like pattern or a saw-tooth-like pattern. In some embodiments,the trenches 303 may form a continuous trench surrounding the unremovedportions 305 of the HA structure 301 as viewed from top, where theunremoved portions 305 of the HA structure 301 have conical shapesarranged in a rectangular array. The pattern of the HA structure 301 asillustrated in FIG. 3 is provided for illustrative purposes only and arenot meant to limit the present invention in any manner. As describedbelow in greater detail, the pattern of the HA structure 301 may bealtered by changing the width W₁, the spacing S₁, and the pitch P₁ ofthe unremoved portions 207 of the patterned mask 203, and by changingparameters of the etching process such as, for example, a duration ofthe etching process. Such patterns of various HA structures areillustrated in FIGS. 9A-13B and are discussed in greater detail below.

FIG. 3 shows that there are about three unremoved portions 305 of the HAstructure 301 per active photosensitive pixel 109A in the illustratedcross section. However, in other embodiments, the number of unremovedportions 305 of the HA structure 301 per active photosensitive pixel109A may be greater than three or less than three, and may varyaccording to design requirements for the BSI image sensor 100.Furthermore, in the illustrated embodiment, top surfaces of unremovedportions 305 of the HA structure 301 have sharp corners. However, inother embodiments, the unremoved portions 305 of the HA structure 301may have rounded or flat top surfaces as discussed in greater detailbelow with reference to FIGS. 9A-13B.

Referring to FIG. 4, after forming the HA structure 301, a bottomanti-reflective coating (BARC) layer 401 is formed on the back surface107B of the BSI image sensor 100. In some embodiments, the BARC layer401 comprises suitable dielectric materials such as silicon oxide,silicon nitride, silicon oxynitride, hafnium oxide, tantalum oxide, or acombination thereof, although other materials may be used. In someembodiments, the BARC layer 401 may formed using atomic layer deposition(ALD), CVD, PECVD, physical vapor deposition (PVD), metal-organicchemical vapor deposition (MOCVD), or the like. In some embodiments inwhich the BARC layer 401 comprises an oxide layer, a low-temperatureremote plasma-assisted oxidation (LRPO) process may be used to form theBARC layer 401. The BARC layer 401 is used to further reduce areflection of incident light from the back surface 107B of the BSI imagesensor 100. In the illustrated embodiment, the BARC layer 401 comprisesa HfO₂ layer having a thickness between about 20 Å and about 150 Å, suchas about 60 Å, and a Ta₂O₅ layer having a thickness between about 350 Åand about 800 Å, such as about 520 Å, where Ta₂O₅ layer is formed overthe HfO₂ layer.

Referring to FIG. 5, a first dielectric layer 501 is formed over theBARC layer 401. In some embodiments, the first dielectric layer 501 maybe formed of silicon oxide, for example, although other dielectricmaterials may be used. In some embodiments, the first dielectric layer501 may be formed using ALD, CVD, PECVD, the like, or a combinationthereof. In some embodiments in which the first dielectric layer 501comprises an oxide layer, LRPO may be used to form the first dielectriclayer 501. In the illustrated embodiment, the first dielectric layer 501is a plasma enhanced oxide (PEOX) layer. Subsequently, the firstdielectric layer 501 is planarized using a grinding process, a chemicalmechanical polishing (CMP) process, an etching process, or the like.After the planarization process, the first dielectric layer 501 may havea thickness between about 250 Å and about 2500 Å, such as about 1300 Å.The first dielectric layer 501 may be also referred to as a first bufferoxide layer 501.

Referring to FIGS. 6 and 7, a metal shield 701 and a metal grid 703 isformed over the first dielectric layer 501 such that the metal shield701 is formed in the BLC region 105 of the BSI image sensor 100 and themetal grid 703 is formed in the pixel array region 103 of the BSI imagesensor 100. Referring first to FIG. 6, a barrier/adhesion layer 601 isformed over the first dielectric layer 501 and a conductive layer 603 isformed over the barrier/adhesion layer 601. In some embodiments, thebarrier/adhesion layer 601 may comprise titanium, titanium nitride,tantalum, tantalum nitride, or multilayers thereof and may be formedusing CVD, PVD, MOCVD, or the like. A conductive layer 603 may comprisealuminum, copper, nickel, tungsten, alloys thereof, or the like and maybe formed using PDV, plating, or the like. In some embodiments, athickness of the barrier/adhesion layer 601 may be between about 50 Åand about 500 Å, and a thickness of the conductive layer 603 may bebetween about 500 Å and about 3500 Å. In the illustrated embodiment, thebarrier/adhesion layer 601 comprises titanium nitride having a thicknessof about 300 Å and the conductive layer 603 comprises tungsten having athickness of about 2000 Å. It is appreciated that the dimensions recitedthroughout the description are merely examples, and may be changed todifferent values.

In some embodiments, the barrier/adhesion layer 601 and the conductivelayer 603 are formed such that the barrier/adhesion layer 601 and theconductive layer 603 are electrically coupled to the substrate 107. Suchelectrical coupling provides proper grounding to the barrier/adhesionlayer 601 and the conductive layer 603, and to the metal shield 701 andthe metal grid 703 that are subsequently formed by patterning thebarrier/adhesion layer 601 and the conductive layer 603. In suchembodiments, the BARC layer 401 and the first dielectric layer 501 arepatterned to form one or more openings that expose the back surface 107Bof the substrate 107 in the BLC region 105. In some embodiments, theBARC layer 401 and the first dielectric layer 501 may be patterned usingsuitable photolithography and etching processes. The barrier/adhesionlayer 601 and the conductive layer 603 fill the one or more openings andare electrically coupled to the substrate 107.

Referring to FIG. 7, the barrier/adhesion layer 601 and the conductivelayer 603 are patterned to form the metal shield 701 and the metal grid703. The patterning process forms a plurality of openings 705 in thebarrier/adhesion layer 601 and the conductive layer 603 in the pixelarray region 103 such that the first dielectric layer 501 is exposedthrough the openings 705. In some embodiments, the active photosensitivepixels 109A are aligned to respective openings 705. A portion of thebarrier/adhesion layer 601 and the conductive layer 603 remaining in theBLC region 105 is used as the metal shield 701. The metal shield 701blocks the light that otherwise would be received by the referencephotosensitive pixels 109B. Portions of the barrier/adhesion layer 601and the conductive layer 603 remaining in the pixel array region 103form the metal grid 703. The metal grid 703 prevents optical cross-talkbetween neighboring active photosensitive pixels 109A. In someembodiments, walls of the metal grid 703 may encircle each activephotosensitive pixel 109A as viewed from top. Furthermore, the metalgrid 703 and the metal shield 701 may be electrically coupled to eachother.

Referring to FIG. 8, a second dielectric layer 801 is formed over themetal grid 703 and the metal shield 701 and fills the openings 705. Insome embodiments, the second dielectric layer 801 may be formed usingsimilar materials and methods as the first dielectric layer 501described above with reference to FIG. 5 and the description is notrepeated herein. In some embodiments, the first dielectric layer 501 andthe second dielectric layer 801 may be formed of a same material. Inother embodiments, the first dielectric layer 501 and the seconddielectric layer 801 may be formed of different materials. Subsequently,the second dielectric layer 801 is planarized using a grinding process,a chemical mechanical polishing (CMP) process, an etching process, orthe like. After the planarization process, the second dielectric layer801 may have a thickness between about 2000 Å and about 5000 Å, such asabout 3400 Å. The second dielectric layer 801 may be also referred to asa second buffer oxide layer 801.

Referring further to FIG. 8, a color filter layer 803 is formed over thesecond dielectric layer 801. In some embodiments, the color filter layer803 comprises a third dielectric layer 805, for example, a silicon oxidelayer with a plurality of color filters 807 formed therein. In someembodiments, the color filters 807 are aligned with respective activephotosensitive pixels 109A. The color filters 807 may be used to allowspecific wavelengths of light to pass while reflecting otherwavelengths, thereby allowing the BSI image sensor 100 to determine thecolor of the light being received by the active photosensitive pixels109A. The color filters 807 may vary, such as being a red, green, andblue filter as used in a Bayer pattern. Other combinations, such ascyan, yellow, and magenta, may also be used. The number of differentcolors of the color filters 807 may also vary. The color filters 807 maycomprise a polymeric material or resin, such as polymethyl-methacrylate(PMMA), polyglycidyl-methacrylate (PGMA), or the like, that includescolored pigments. In some embodiments, reflective guide layers (notshown) are formed along sidewalls of the color filters 807. Thereflective guide layers are formed of a metal or other high refractiveindex material that is capable of reflecting light, such as copper,aluminum, tantalum nitride, titanium nitride, tungsten, silicon nitride,the like, or a combination thereof.

An array of microlenses 809 is formed over the color filter layer 803.In some embodiments, the microlenses 809 are aligned with respectivecolor filters 807 and respective active photosensitive pixels 109A. Themicrolenses 809 may be formed of any material that may be patterned andformed into lenses, such as a high transmittance acrylic polymer. In anembodiment, a microlens layer may be formed using a material in a liquidstate by, for example, spin-on techniques. Other methods, such as CVD,PVD, or the like, may also be used. The planar material for themicrolens layer may be patterned using suitable photolithography andetching methods to pattern the planar material in an array correspondingto the array of the active photosensitive devices. The planar materialmay then be reflowed to form an appropriate curved surface for themicrolenses 809. Subsequently, the microlenses 809 may be cured using,for example, a UV treatment. In some embodiments, after forming themicrolenses 809, the carrier substrate 201 may be de-bonded form the BSIimage sensor 100 and the BSI image sensor 100 may undergo furtherprocessing such as, for example, packaging.

FIGS. 9A and 9B are scanning electron microscope (SEM) imagesillustrating a perspective and a cross-sectional views, respectively, ofa HA structure 900 formed on a back surface of a substrate 901 inaccordance with some embodiments. The HA structure 900 may be formedusing similar methods as the HA structure 301 described above withreference to FIGS. 2 and 3 and description is not repeated herein. Inthe illustrated embodiment, the substrate 901 formed of silicon ispatterned using a wet etch with TMAH for a duration of about 60 s toform a plurality of trenches 903. In the illustrated embodiment, thetrenches 903 have a height of about 5735 Å. The unremoved features 905of the HA structure 900 that have a conical shape are arranged in arectangular array, such that the trenches 903 form a continuous trenchsurrounding each of the unremoved features 905 of the HA structure 900.The patterned mask (not illustrated) similar to the patterned mask 203illustrated in FIG. 2 is used to aid in patterning the substrate 901. Inthe illustrated embodiments, unremoved features of the patterned mask(similar to the unremoved portions 207 of the patterned mask 203) have awidth of about 0.1 μm, a spacing of about 0.4 μm, and a pitch of about0.5 μm.

FIGS. 10A and 10B are scanning electron microscope (SEM) imagesillustrating a perspective and a cross-sectional views, respectively, ofa HA structure 1000 formed on a back surface of a substrate 1001 inaccordance with some embodiments. The HA structure 1000 may be formedusing similar methods as the HA structure 301 described above withreference to FIGS. 2 and 3 and description is not repeated herein. Inthe illustrated embodiment, the back surface of the substrate 1001 thatis formed of silicon is patterned using a wet etch with TMAH for aduration of about 90 s to form a plurality of trenches 1003. In theillustrated embodiment, the trenches 1003 have a height of about 5259 Å.The unremoved features 1005 of the HA structure 1000 that have a conicalshape are arranged in a rectangular array, such that plurality oftrenches 1003 form a continuous trench surrounding each of the unremovedfeatures 1005 of the HA structure 1000. The patterned mask (notillustrated) similar to the patterned mask 203 illustrated in FIG. 2 isused to aid in patterning the substrate 1001. In the illustratedembodiments, unremoved features of the patterned mask (similar to theunremoved portions 207 of the patterned mask 203) have a width of about0.1 μm, a spacing of about 0.55 μm, and a pitch of about 0.65 μm.

FIGS. 11A and 11B are scanning electron microscope (SEM) imagesillustrating a perspective and a cross-sectional views, respectively, ofa HA structure 1100 formed on a back surface of a substrate 1101 inaccordance with some embodiments. In the illustrated embodiment, theunremoved features 1105 of the HA structure 1100 form a continuousfeature having a shape of a rectangular grid and each of the openings1103 is surrounded by walls of the rectangular grid. The HA structure1100 may be formed using similar methods as the HA structure 301described above with reference to FIGS. 2 and 3 and description is notrepeated herein. In the illustrated embodiment, the back surface of thesubstrate 1101 that is formed of silicon is patterned using a wet etchwith TMAH for a duration of about 60 s to form a plurality of openings1103. The patterned mask (not illustrated) similar to the patterned mask203 illustrated in FIG. 2 is used to aid in patterning the substrate1101. In the illustrated embodiments, unremoved features of thepatterned mask (similar to the unremoved portions 207 of the patternedmask 203) have a width of about 0.3 μm, a spacing of about 0.15 μm, anda pitch of about 0.45 μm.

FIGS. 12A and 12B are scanning electron microscope (SEM) imagesillustrating a perspective and a cross-sectional views, respectively, ofa HA structure 1200 formed on a back surface of a substrate 1201 inaccordance with some embodiments. The HA structure 1200 may be formedusing similar methods as the HA structure 301 described above withreference to FIGS. 2 and 3 and description is not repeated herein. Inthe illustrated embodiment, the back surface of the substrate 1201 thatis formed of silicon is patterned using a wet etch with TMAH for aduration of about 60 s to form a plurality of trenches 1203. In theillustrated embodiment, the trenches 1203 have a height of about 5755 Å.The unremoved features 1205 of the HA structure 1200 that have a conicalshape are arranged in a rectangular array, such that plurality oftrenches 1203 form a continuous trench surrounding each of the unremovedfeatures 1205 of the HA structure 1200. The patterned mask (notillustrated) similar to the patterned mask 203 illustrated in FIG. 2 isused to aid in patterning the substrate 1201. In the illustratedembodiments, unremoved features of the patterned mask (similar to theunremoved portions 207 of the patterned mask 203) have a width of about0.2 μm, a spacing of about 0.4 μm, and a pitch of about 0.6 μm.

FIGS. 13A and 13B are scanning electron microscope (SEM) imagesillustrating a perspective and a cross-sectional views, respectively, ofa HA structure 1300 formed on a back surface of a substrate 1301 inaccordance with some embodiments. The HA structure 1300 may be formedusing similar methods as the HA structure 301 described above withreference to FIGS. 2 and 3 and description is not repeated herein. Inthe illustrated embodiment, the back surface of the substrate 1301 thatis formed of silicon is patterned using a wet etch with TMAH for aduration of about 60 s to form a plurality of trenches 1303. In theillustrated embodiment, the trenches 1303 have a height of about 5636 Å.The unremoved features 1305 of the HA structure 1200 that have acylindrical shape are arranged in a rectangular array, such thatplurality of trenches 1303 form a continuous trench surrounding each ofthe unremoved features 1305 of the HA structure 1300. The patterned mask(not illustrated) similar to the patterned mask 203 illustrated in FIG.2 is used to aid in patterning the substrate 1301. In the illustratedembodiments, unremoved features of the patterned mask (similar to theunremoved portions 207 of the patterned mask 203) have a width of about0.36 μm, a spacing of about 0.24 μm, and a pitch of about 0.6 μm.

In some embodiments, by forming an HA structure (such as, for example,the HA structures 301, 900, 1000, 1100, 1200 and 1300) on a back surfaceof a substrate, a reflectance of the back surface of the substrate maybe reduced compared to an unpatterned portion of the back surface of thesubstrate. FIG. 14 depicts various curves illustrating the dependence ofthe reflectance on a wavelength of incident light that forms an angle ofabout 15 degrees with a normal to the back surface of the substrate. Thecurve 1401 is a reflectance of a back surface of an unpatentedsubstrate. The curves 1403 and 1405 are reflectances of back surfaces ofsubstrates having HA structures. As show in FIG. 14, the HA structuresmay significantly reduce the reflectance of the back surface of thesubstrate. In some embodiments, the reflectance of the back surface ofthe substrate may be reduced by about 30% or greater by forming an HAstructure on the back surface of the substrate. Furthermore, thereflectance of light is reduced not only in the visible region but alsoin the near infrared (NIR) region, which advantageously allows forapplication of the HA patterns in NIR devices.

FIG. 15 is a flow diagram illustrating a method 1500 of forming a BSIimage sensor in accordance with some embodiments. The method starts withstep 1501, where one or more photosensitive regions (such as thephotosensitive pixel 109 illustrated in FIG. 1) are formed on a firstside of a substrate (such as the front surface 107A of the substrate 107illustrated in FIG. 1) as described above with reference to FIG. 1. Instep 1503, a high absorption (HA) structure (such as the HA structure301 illustrated in FIG. 3) is formed on a second side of the substrate(such as the back surface 107B of the substrate 107 illustrated inFIG. 1) that is opposite the first side of the substrate as describedabove with reference to FIGS. 2 and 3. In step 1505, a metal grid (suchas the metal grid 703 illustrated in FIG. 7) is formed over the HAstructure as described above with reference to FIGS. 4-7.

FIGS. 16-19 are cross-sectional views of various processing steps duringfabrication of a BSI image sensor 1600 in accordance with someembodiments. FIG. 16 illustrates an intermediate structure of the BSIimage sensor 1600 in accordance with some embodiments. In someembodiments, the intermediate structure of the BSI image sensor 1600 issimilar to the intermediate structure of the BSI image sensor 100described above with reference to FIG. 1, with like elements labeled bylike reference numbers, and the description is not repeated herein.

Referring FIG. 17, the BSI image sensor 1600 is flipped and bonded to acarrier substrate 201 such that the front surface 107A of the substrate107 faces the carrier substrate 201 and the back surface 107B of thesubstrate 107 is exposed for further processing. In some embodiments,the BSI image sensor 1600 is bonded to the carrier substrate 201 usingsimilar methods as described above with reference to FIG. 2 and thedescription is not repeated herein. After the BSI image sensor 1600 andthe carrier substrate 201 are bonded, a thinning process may be appliedto the back surface 107B of the BSI image sensor 1600. In someembodiments, the back surface 107B of the BSI image sensor 1600 may bethinned using similar methods as described above with reference to FIG.2 and the description is not repeated herein.

After thinning, a first dielectric layer 1701 is formed on the backsurface 107B of the substrate 107. In some embodiments, the firstdielectric layer 1701 may be formed using similar materials and methodsas the first dielectric layer 501 described above with reference to FIG.5 and the description is not repeated herein. In some embodiments, athickness of the first dielectric layer 1701 may be between about 50 Aand about 250 A, such as about 100 A. In some embodiments, the thicknessof the first dielectric layer 1701 may be less than the thickness of thefirst dielectric layer 501, which may advantageously allow reduction ofthe optical path of the BSI image sensor 1600 compare to the BSI imagesensor 100. The first dielectric layer 1701 may also referred to as afirst buffer oxide layer 1701. Accordingly, the quantum efficiency ofthe BSI image sensor 1600 may be advantageously increased and theoptical cross-talk of the BSI image sensor 1600 may be advantageouslyreduced.

Subsequently, a barrier/adhesion layer 1703 is formed over the firstdielectric layer 1701 and a conductive layer 1705 is formed over thebarrier/adhesion layer 1703. In some embodiments, the barrier/adhesionlayer 1703 and the conductive layer 1705 may be formed using similarmaterials and methods as the barrier/adhesion layer 601 and theconductive layer 603, respectively, described above with reference toFIG. 6 and the description is not repeated herein. In some embodiments,a thickness of the barrier/adhesion layer 1703 may be between about 100Å and about 500 Å, and a thickness of the conductive layer 1705 may bebetween about 500 Å and about 3000 Å. In the illustrated embodiment, thebarrier/adhesion layer 1703 comprises titanium nitride having athickness of about 300 Å and the conductive layer 1705 comprisestungsten having a thickness of about 2000 Å. It is appreciated that thedimensions recited throughout the description are merely examples, andmay be changed to different values. As described below in greaterdetail, the barrier/adhesion layer 1703 and the conductive layer 1705are patterned to form a metal shield 1801 and a metal grid 1803 (seeFIG. 18).

In some embodiments, a second dielectric layer 1707 is formed over theconductive layer 1705. In some embodiments, the second dielectric layer1707 may be formed using similar materials and methods as the firstdielectric layer 501 described above with reference to FIG. 5 and thedescription is not repeated herein. In some embodiments, the firstdielectric layer 1701 and the second dielectric layer 1707 may be formedof a same material. In other embodiments, the first dielectric layer1701 and the second dielectric layer 1707 may be formed of differentmaterials. Subsequently, the second dielectric layer 1707 is planarizedusing a grinding process, a CMP process, an etching process, or thelike. After the planarization process, the second dielectric layer 1707may have a thickness between about 500 Å and about 2000 Å. The seconddielectric layer 1707 may be also referred to as a second buffer oxidelayer 1707.

Referring further to FIG. 17, a patterned mask 1709 is formed over thesecond dielectric layer 1707. In some embodiments, the patterned mask1709 may be formed using similar materials and methods as the patternedmask 203 described above with reference to FIG. 2 and the description isnot repeated herein. In some embodiments, the patterned mask 1709comprises a plurality of opening 1711 exposing portions of the seconddielectric layer 1707 in the pixel array region 103, while the seconddielectric layer 1707 in the BLC region is completely covered by thepatterned mask 1709. In some embodiments, unremoved portions 1713 of thepatterned mask 1709 have a uniform width W₂, uniform spacing S₂, and auniform pitch P₂, with P₂ being equal to a sum of W₂ and S₂. In someembodiments, the unremoved portions 1713 of the patterned mask 1709 mayform a two-dimensional rectangular array as viewed from top. Asdescribed below in greater detail, the patterned mask 1709 is used as anetch mask while patterning the first dielectric layer 1701, thebarrier/adhesion layer 1703, the conductive layer 1705, and the seconddielectric layer 1707. In some embodiments, the width W₂ is betweenabout 0.08 μm and about 0.3 μm, the spacing S₂ is between about 0.72 μmand about 2.7 μm, and the pitch P₂ is between about 0.8 μm and about 3μm.

Referring to the FIG. 18, the first dielectric layer 1701, thebarrier/adhesion layer 1703, the conductive layer 1705, and the seconddielectric layer 1707 are patterned to form a plurality of openings1805. The openings 1805 extend through the first dielectric layer 1701,the barrier/adhesion layer 1703, the conductive layer 1705 and thesecond dielectric layer 1707, and expose portions of the back surface107B of the substrate 107 in the pixel array region 103. In someembodiments, the first dielectric layer 1701, the barrier/adhesion layer1703, the conductive layer 1705 and the second dielectric layer 1707 maybe patterned using one or multiple suitable etching processes, whileusing the patterned mask 1709 as an etch mask. After patterning, thepatterned mask 1709 is removed. In some embodiments, the patterned mask1709 may be removed using similar methods as the patterned mask 203described above with reference to FIG. 3 and the description is notrepeated herein.

Referring further to FIG. 18, the patterning process forms the metalshield 1801 and the metal grid 1803. In the illustrated embodiments,walls of the metal grid 1803 may have a width, a spacing, a pitchsimilar to the width W₂, the spacing S₂, and the pitch P₂, respectively,of the unremoved portions 1713 of the patterned mask 1709. As describedin greater detail below, the metal grid 1803, with respective unremovedportions of the first dielectric layer 1701 and the second dielectriclayer 1707, is used a mask to aid in patterning the back surface 107B ofthe substrate 107 to form an HA structure 1901 (see FIG. 19). In someembodiments, a third dielectric layer 1807 is formed over the metal grid1803 and the metal shield 1801 to protect the metal grid 1803 frompatterning process while forming the HA structure 1901. In someembodiments, the third dielectric layer 1807 may be formed using similarmaterials and methods as the first dielectric layer 501 described abovewith reference to FIG. 5 and the description is not repeated herein. Insome embodiments, the first dielectric layer 1701, the second dielectriclayer 1707 and the third dielectric layer 1807 may be formed of a samematerial. In other embodiments, the first dielectric layer 1701, thesecond dielectric layer 1707 and the third dielectric layer 1807 may beformed of different materials.

In some embodiments, a patterned mask 1809 is formed over the thirddielectric layer 1807. In the illustrated embodiment, the patterned mask1809 protects the metal shield 1801 in the BLC region 105 of the BSIimage sensor 1600, while the metal grid 1803 in the pixel array region103 of the BSI image sensor 1600 is exposed. In some embodiments, thepatterned mask 1809 may be formed using similar materials and methods asthe patterned mask 203 described above with reference to FIG. 2 and thedescription is not repeated herein. As described below in greaterdetail, the metal grid 1803 and the patterned mask 1809 is used acombined mask to aid in patterning the back surface 107B of thesubstrate 107.

Referring to FIG. 19, the back surface 107B of the substrate 107 in thepixel array region 103 is patterned to form the HA structure 1901. Insome embodiments, the back surface 107B of the substrate 107 ispatterned using a suitable etching process while using the metal grid1803 and the patterned mask layer 1809 as a combined etch mask. In someembodiments, the HA structure 1901 may be formed using similar etchingprocesses as the HA structure 301 described above with reference to FIG.3 and the description is not repeated herein. In the illustratedembodiment, the HA structure 1901 comprises a plurality of trenches 1903and a plurality of unremoved features 1905 that are interposed betweenneighboring trenches 1903. In some embodiments, the HA structure 1901may have a pattern as described above with reference to FIGS. 9A-13B andthe description is not repeated herein. In some embodiments, thepatterning process that forms the HA structure 1901 may also remove afirst portion of the third dielectric layer 1807 in the pixel arrayregion 103, while a second portion of the third dielectric layer 1807 inthe BLC region 105 remains unremoved. Subsequently, the patterned mask1809 is removed using similar methods as the patterned mask 203described above with reference to FIG. 2 and the description is notrepeated herein.

Referring further to FIG. 19, after forming the HA structure 1901, aBARC layer 1907 is formed on sidewalls of the trenches 1903 and over themetal grid 1803 and the metal shield 1801. In some embodiments, the BARClayer 1907 may be formed using similar materials and methods as the BARClayer 401 described above with reference to FIG. 4 and the descriptionis not repeated herein. In the illustrated embodiment, the thirddielectric layer 1807 is removed in the pixel array region 103 and atleast a portion of the BARC layer 1907 contacts the sidewalls of thetrenches 1903 and sidewalls of the metal grid 1803.

In some embodiments, a fourth dielectric layer 1909 is formed over theBARC layer 1907. In some embodiments, the fourth dielectric layer 1909may be formed using similar materials and methods as the firstdielectric layer 501 described above with reference to FIG. 5 and thedescription is not repeated herein. In some embodiments, the firstdielectric layer 1701, the second dielectric layer 1707, the thirddielectric layer 1807, and the fourth dielectric layer 1909 may beformed of a same material. In other embodiments, the first dielectriclayer 1701, the second dielectric layer 1707, the third dielectric layer1807, and the fourth dielectric layer 1909 may be formed of differentmaterials. Subsequently, the fourth dielectric layer 1909 is planarizedusing a grinding process, a CMP process, an etching process, or thelike. After the planarization process, the fourth dielectric layer 1909may have a thickness between about 500 Å and about 3000 Å, such as about1500 Å. The fourth dielectric layer 1909 may be also referred to as afourth buffer oxide layer 1909.

Referring further to FIG. 19, a color filter layer 1911 is formed overthe fourth dielectric layer 1909. In some embodiments, the color filterlayer 1911 comprises a fifth dielectric layer 1913 with a plurality ofcolor filters 1915 formed therein. In some embodiments, the fifthdielectric layer 1913 and the color filters 1915 may formed usingsimilar materials and methods as the third dielectric layer 805 and thecolor filters 807, respectively, described above with reference to FIG.8 and the description is not repeated herein. Subsequently, microlenses1917 are formed over the color filter layer 1911, such that themicrolenses 1917 are aligned with respective color filters 1915 andrespective active photosensitive pixels 109A. In some embodiments, themicrolenses 1917 may be formed using similar materials and methods asthe microlenses 809 described above with reference to FIG. 8 and thedescription is not repeated herein. In some embodiments, after formingthe microlenses 1917, the carrier substrate 201 may be de-bonded formthe BSI image sensor 1600 and the BSI image sensor 1600 may undergofurther processing such as, for example, packaging.

FIG. 20 is a flow diagram illustrating a method 2000 of forming a BSIimage sensor in accordance with some embodiments. The method 2000 startswith step 2001, where one or more photosensitive regions (such as thephotosensitive pixel 109 illustrated in FIG. 16) are formed on a firstside of a substrate (such as the front surface 107A of the substrate 107illustrated in FIG. 16) as described above with reference to FIG. 16. Instep 2003, a metal grid (such as the metal grid 1803 illustrated in FIG.18) is formed over a second side of the substrate (such as the backsurface 107B of the substrate 107 illustrated in FIG. 16) that isopposite the first side of the substrate as described above withreference to FIGS. 17 and 18. In step 2003, after forming the metalgrid, a high absorption (HA) structure (such as the HA structure 1901illustrated in FIG. 19) is formed on the second side of the substrate asdescribed above with reference to FIG. 19.

Various embodiments presented herein may provide several advantages.Embodiments such as described herein allow for forming a BSI imagesensor having a high absorption (HA) structure on a backside. By formingthe HA structure on a backside of a BSI image sensor, light reflectancefrom the back side may be improved. Accordingly, performance of the BSIimage sensor may be advantageously improved in visible and near infraredregimes. Furthermore, various embodiments described herein reduce theoptical path of a BSI image sensor, which advantageous improves thequantum efficiency and reduces the optical cross-talk of the BSI imagesensor.

According to an embodiment, a method includes forming a plurality ofphotosensitive pixels in a substrate, the substrate having a firstsurface and a second surface, the second surface being opposite thefirst surface, the substrate having one or more active devices on thefirst surface. A first portion of the second surface is protected. Asecond portion of the second surface is patterned to form recesses inthe substrate. An anti-reflective layer is formed on sidewalls of therecesses. A metal grid is formed over the second portion of the secondsurface, the anti-reflective layer being interposed between thesubstrate and the metal grid.

According to another embodiment, a method includes forming a pluralityof photosensitive regions in a substrate, the substrate having a firstsurface and a second surface, the second surface being opposite thefirst surface, the substrate having at least one active device on thefirst surface. A metal grid is formed over a first portion of the secondsurface, the metal grid exposing portions of the substrate. Exposedportions of the substrate are recessed to form recesses in thesubstrate. An anti-reflective layer is formed on sidewalls of therecesses and sidewalls of the metal grid. The recesses are filled with afirst dielectric layer, the first dielectric layer extending above atopmost surface of the metal grid.

According to yet another embodiment, a device includes a substrate, thesubstrate having a first surface and a second surface, the first surfacebeing opposite the second surface, at least one active device on thefirst surface of the substrate, and a plurality of photosensitiveregions in the substrate. The device further includes a plurality ofrecesses in the second surface of the substrate, the plurality ofrecesses being disposed over the plurality of photosensitive regions, ananti-reflective layer extending along sidewalls of the plurality ofrecesses, and a metal grid over the plurality of recesses.

According to yet another embodiment, a method includes forming aplurality of photosensitive devices in a substrate, the substrate havinga pixel array region and a black level correction (BLC) region adjacentthe pixel array region. An interconnect structure is formed on a firstside of the substrate. A second side of the substrate is patterned inthe pixel array region to form recesses in the substrate, the secondside being opposite the first side. An anti-reflective layer is blanketdeposited over the recesses in the pixel array region and over thesecond side of the substrate in the BLC region. A first dielectric layeris formed over the anti-reflective layer, the first dielectric layeroverfilling the recesses. A metal layer is formed over the firstdielectric layer. The metal layer is patterned to form a metal grid overthe pixel array region and a metal shield over the BLC region.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a plurality ofphotosensitive pixels in a substrate, the substrate having a firstsurface and a second surface, the second surface being opposite thefirst surface, the substrate having one or more active devices on thefirst surface; protecting a first portion of the second surface;patterning a second portion of the second surface to form recesses inthe substrate; forming an anti-reflective layer on sidewalls of therecesses; and forming a metal grid over the second portion of the secondsurface, the anti-reflective layer being interposed between thesubstrate and the metal grid.
 2. The method of claim 1, furthercomprising forming a metal shield over the first portion of the secondsurface, the anti-reflective layer being interposed between thesubstrate and the metal shield.
 3. The method of claim 1, whereinforming the metal grid comprises: forming a metal layer over the secondportion of the second surface; and patterning the metal layer to formopenings therein, the openings being aligned with respectivephotosensitive pixels.
 4. The method of claim 3, further comprising,after patterning the second portion of the second surface, filling theopenings with a dielectric material.
 5. The method of claim 3, furthercomprising, prior to forming the metal layer, filling the recesses witha dielectric material, the anti-reflective layer being interposedbetween the sidewalls of the recesses and the dielectric material. 6.The method of claim 1, wherein patterning the second portion of thesecond surface comprises: forming a patterned mask on the second portionof the second surface, the patterned mask exposing portions of thesubstrate; and etching exposed portions of the substrate to form therecesses in the substrate.
 7. The method of claim 1, wherein portions ofthe substrate interposed between neighboring recesses have a uniformwidth and a uniform pitch.
 8. The method of claim 1, wherein thesidewalls of the recesses are slanted.
 9. A method comprising: forming aplurality of photosensitive devices in a substrate, the substrate havinga pixel array region and a black level correction (BLC) region adjacentthe pixel array region; forming an interconnect structure on a firstside of the substrate; patterning a second side of the substrate in thepixel array region to form recesses in the substrate, the second sidebeing opposite the first side; blanket depositing an anti-reflectivelayer over the recesses in the pixel array region and over the secondside of the substrate in the BLC region; forming a first dielectriclayer over the anti-reflective layer, the first dielectric layeroverfilling the recesses; forming a metal layer over the firstdielectric layer; and patterning the metal layer to form a metal gridover the pixel array region and a metal shield over the BLC region. 10.The method of claim 9, wherein at least a portion of the metal shieldextends into the first dielectric layer.
 11. The method of claim 9,wherein sidewalls of the recesses form a saw-tooth pattern in across-sectional view.
 12. The method of claim 9, further comprisingforming a second dielectric layer over the metal grid and the metalshield, the second dielectric layer extending along sidewalls of themetal grid and a sidewall of the metal shield.
 13. The method of claim9, wherein blanket depositing the anti-reflective layer over therecesses in the pixel array region and over the second surface of thesubstrate in the BLC region comprises: blanket depositing a firstsublayer over the recesses in the pixel array region and over the secondsurface of the substrate in the BLC region, the first sublayercomprising a first oxide material; and blanket depositing a secondsublayer over the first sublayer, the second sublayer comprising asecond oxide material different from the first oxide material.
 14. Themethod of claim 9, further comprising performing a planarization processon the first dielectric layer.
 15. The method of claim 9, furthercomprising, before forming the metal layer over the first dielectriclayer, forming a barrier layer over the first dielectric layer.
 16. Adevice comprising: a substrate, the substrate having a first surface anda second surface, the first surface being opposite the second surface;at least one active device on the first surface of the substrate; aplurality of photosensitive regions in the substrate; a plurality ofrecesses in the second surface of the substrate, the plurality ofrecesses being disposed over the plurality of photosensitive regions; ananti-reflective layer extending along sidewalls of the plurality ofrecesses; and a metal grid over the plurality of recesses.
 17. Thedevice of claim 16, further comprising a metal shield adjacent the metalgrid, the metal shield being substantially level with the metal grid.18. The device of claim 17, wherein the anti-reflective layer extendsalong a sidewall of the metal shield and sidewalls of the metal grid.19. The device of claim 16, wherein portions of the substrate interposedbetween neighboring recesses are arranged in a rectangular array asviewed from top.
 20. The device of claim 16, wherein portions of thesubstrate interposed between neighboring recesses form a rectangulargrid as viewed from top.